The present invention relates to modulation schemes for video telephones using quadrature amplitude modulation.
When a single channel of data is transmitted over a telephone line using a sine wave signal, the zero crossings of the sine wave can be used for synchronization. Such a system is shown in U.S. Pat. No. 4,739,413 which describes a video-optimized modulation scheme of Luma Telecom, Inc. The amplitude and phase of each cycle of a sine wave is modulated to transmit the video gray scale or luminance information. This modulation does not affect the zero crossings in between symbols, which can be used by a simple chopper demodulator to recover the carrier.
The amount of black and white data transmitted can be increased by adding another channel using quadrature amplitude modulation (QAM). This allows more gray scale data to be sent or allows color information to be sent. However, because the two channels of the QAM signal are out of phase, the zero crossings of each channel will be altered by the data on the other channel. Thus, a more complicated demodulation scheme is typically necessary.
It is desirable to have a QAM system which can provide an increased amount of luminance data (gray scale) or color data and would be compatible with existing demodulators which use a chopper demodulator for a single channel for black and white (gray scale) pictures.